1. Field of the Invention
The present invention relates to a light-emitting display device that employs light-emitting elements such as organic EL (electroluminescent) elements and a driving method therefor.
2. Description of Related Art
In recent years, organic EL elements that are self-light-emitting elements employing organic compounds have been extensively studied, and dot matrix displays employing an organic EL element have been developed as well.
FIG. 1 shows an equivalent circuit of an organic EL element. FIG. 2A shows the current luminance properties of the organic EL element, FIG. 2B shows the voltage-current properties of the organic EL element, and FIG. 2C shows the voltage luminance properties.
As shown in FIG. 1, the organic EL element can be represented by a light-emitting element E having diode properties, and the parasitic capacitance C connected in parallel to the light-emitting element E and the resistance R connected in series with the light-emitting element E.
As shown in FIGS. 2A through 2C, the organic EL element emits light with luminance in proportion to current. In the case where the driving voltage is less than the predetermined light emission specifying voltage Vth, it allows current to hardly flow, resulting in practically no emission.
FIG. 3 shows a driving method of a prior art light-emitting element.
The driving method shown in FIG. 3 is called the passive matrix driving method, in which the positive electrode lines A1 through A4 and the negative electrode lines B1 through Bn (n is a natural number. Four positive electrode lines are used for ease of explanation) are arranged in a matrix (grid). To each intersection of the positive electrode lines and the negative electrode lines arranged in a matrix, light-emitting elements E11 through E4n are connected. Either one of the positive electrode lines or the negative electrode lines are selected for scanning at constant intervals of time and other lines are driven by the constant-current sources 21 through 24, whereby light-emitting elements at arbitrary intersections are allowed for emitting light in synchronization with the scanning.
A voltage source may be used for the driving source, however, a current source may be preferably used to provide better reproducibility of luminance. This is because current luminance properties are more stable against changes in environmental temperature than voltage luminance properties, and current luminance properties of light-emitting elements have a linear proportionality.
In the case of FIG. 3, the driving source employs constant-current sources with the amount of constant current sufficient for the desired instantaneous luminance. Therefore, when the instantaneous luminance of light-emitting elements is desired to be equal to Lx, as shown in FIGS. 2A through 2C, the amount of constant current of a driving source is to be set to Ix. Also the voltage across both ends of the light-emitting element (hereinafter designated the light emission specifying voltage) becomes Vx when light is emitted with desired instantaneous luminance (hereinafter designated a steady state of light emission).
There are two driving methods by means of said driving sources, namely, scanning negative electrode lines and driving positive electrode lines, and scanning positive electrode lines and driving negative electrode lines. FIG. 3 shows the method of scanning negative electrode lines and driving positive electrode lines. The negative electrode line scan circuit, 1, is connected to the negative electrode lines B1 through Bn. The positive electrode line drive circuit 2 that comprises the current sources 21 through 24 and the drive switches 31 through 34 are also connected to the positive electrode lines A3 through A4.
The negative electrode line scan circuit 1 performs scanning while sequentially switching the scan switches 11 through 1n over to the ground terminal sides at constant intervals of time, thereby providing negative electrode lines B1 through Bn with ground potential (0V) in sequence. Furthermore, the positive electrode line drive circuit 2 controls the on and off of the drive switches 31 through 34 in synchronization with the switch scanning of said negative electrode line scan circuit 1. This allows the positive electrode lines A1 through A4 to be connected with the constant-current sources 21 through 24 to supply driving current to light-emitting elements located at desired intersections. These negative electrode line scan circuit 1 and the positive electrode line drive circuit 2 are drive-controlled by means of a control circuit that is not shown.
For example, a case where the light-emitting elements E11 and E21 are lit is taken as an example. As shown in the drawing, when the scan switch 11 of the negative electrode line scan circuit 1 is switched to the ground side with the ground potential applied to the first negative electrode line B1, the drive switches 31 and 32 of the positive electrode line drive circuit 2 are preferably switched over to the sides of the constant-current sources to connect the constant-current sources 21 and 22 to the positive electrode lines A1 and A2. By repeating the scanning and driving at a high speed, control is performed in a manner such that light-emitting elements at arbitrary positions are lit as if each light-emitting element emits light at the same time.
Other negative electrode lines B2 through Bn except for negative electrode line B1 that is being scanned are connected with the constant voltage sources 42 through 4n to apply a reverse bias voltage V1 that has the same potential as the light emission specifying voltage Vx. This prevents the light-emitting elements E12 through E1n and E22 through E2n, connected to the positive electrode lines A1 and A2, emitting light accidentally.
The reverse bias voltage sources 41 through 4n, which provide the reverse bias voltage V1, are provided so that light-emitting elements connected to the intersections of the positive electrode lines A1 and A2 to be driven and the negative electrode lines B2 through Bn not to be scanned (E12 through E1n and E22 through E2n in the case of FIG. 3) do not emit light accidentally. Accordingly, the voltage applied thereto is preferably set in a manner such that the voltage across both ends of the light-emitting element is equal to or less than the light emission threshold voltage Vth. However, the reverse bias voltage V1 is best set to the light emission specifying voltage Vx for the reason mentioned below. That is, letting V1=Vx causes the voltage across both ends of the light-emitting element to become 0, and thus the current supplied by the drive source flows only into the light-emitting elements that are emitting light, thereby reproducing a desired luminance in accuracy.
As mentioned above referring to FIG. 3, the state of charge of each parasitic capacitance of each light-emitting element is as follows. The light-emitting elements E11 and E21 connected to the intersections of the positive electrode lines A1 and A2 to be driven and the negative electrode line B1 to be scanned are forward charged. The light-emitting elements E11 through E1n and E22 through E2n connected to the intersections of the positive electrode lines A1 and A2 to be driven and the negative electrode lines B2, B3, and B4, which are not scanned, are not charged. The light-emitting elements E31 and E41 connected to the intersections of the positive electrode lines A3 and A4 not to be driven and the negative electrode line B1 to be scanned are not charged. The light-emitting elements E32 through E3n and E42 through E4n, connected to the intersections of the positive electrode lines A3 and A4, which are not driven, and the negative electrode lines B2, B3, and B4, which are not scanned, are reverse charged. (In the drawing, each light-emitting element E is represented by the symbol of a capacitor, a light-emitting element that is lit is represented by the symbol of a diode, and a capacitor that is charged is shaded.)
This driving method, however, had the following problem caused by parasitic capacitance C in the equivalent circuit of a light-emitting element shown in FIG. 1. The problem will be explained below.
In FIGS. 7A and 7B, the light-emitting elements E11 through E1n connected to said positive electrode line A1 in FIG. 3 are extracted with each of the light-emitting elements E11 through E1n shown only by said parasitic capacitance C. In a case where the positive electrode line A1 is not driven at the time of scanning the negative electrode line B1, the parasitic capacitors C12 through C1n of the light-emitting elements E12 through E1n other than the parasitic capacitor C11 of the light-emitting element E11 connected to the negative electrode line B1 which is currently scanned, are charged by the reverse bias voltage V1 applied to each of the negative electrode lines B2 through Bn which are charged in the direction as shown in FIG. 7A.
When the scanning position is shifted from the negative electrode line B1 to the following negative electrode line B2, the positive electrode line A1 is driven to cause, for example, the light-emitting element E12 to emit light providing the circuit status as shown in FIG. 7B. At the instant circuits are switched over like this, not only is the parasitic capacitor of the light-emitting element E12 that is to be lit charged but also other parasitic capacitors of the light-emitting elements E13 through E1n connected to other negative electrode lines B3 through Bn are charged by letting current flow therein in the direction shown with the arrows.
As mentioned in the foregoing, a light-emitting element is not allowed to emit light with a desired luminance unless the voltage across both ends thereof reaches the light emission specifying voltage Vx. According to the prior art driving method, as shown in FIGS. 7A and 7B in the foregoing, when the positive electrode line A1 is driven to allow the light-emitting element E12 connected to the negative electrode line B2 to emit light, which causes not only the parasitic capacitor of the light-emitting element E12 that to be lit but also other light-emitting elements E13 through E1n that are connected to the positive electrode line A1 to be charged. Thus, until the parasitic capacitors of all these light-emitting elements have been completely charged, the voltage across both ends of the light-emitting element E12 connected to the negative electrode line B2 is not allowed to reach the light emission specifying voltage Vx.
Accordingly, in the prior art driving method, there was a problem in that the rate of rise was slow until light emission was fired and a high-speed scanning could not be performed.
Said problem would exert adverse effects with the increasing number of light-emitting elements. Especially, in the case of employing organic EL elements as light-emitting elements, the effect of said problem would be brought to the fore since organic EL elements have a large parasitic capacitance C due to the surface light emission scheme thereof.
A driving method for solving the aforementioned problem is disclosed in Japanese Patent Kokai No. Hei 9-232074.
The driving method disclosed in said publication will be explained referring to FIG. 3 through FIG. 6. FIG. 3 is a view for explaining the state of light emission A, FIG. 4 is a view for explaining the state of reset, FIG. 5 is view for explaining the transition to the state of light emission B, and FIG. 6 is a view for explaining the state of light emission B.
For explanation, taken as an example is the case of shifting from a state where the light-emitting elements E11 and E12 are lit at the time of scanning the negative electrode line B1, through the reset period shown in FIG. 4, and then to a state where the light-emitting elements E22 and E32 are lit at the time of scanning the negative electrode line B2 as shown in FIG. 5 and FIG. 6.
The point in said publication is that, in the case of allowing the light-emitting elements E22 and E32 to emit light following the light-emitting elements E11 and E21, a reset period is provided for resetting the voltages across both ends of all light-emitting elements E11 through E4n to 0 potential while scanning is switched from the negative electrode line B1 over to the negative electrode line B2 to allow charge accumulated in parasitic capacitors C to be discharged.
That is, as shown in FIG. 4, all scan switches 11 through 1n connected to the negative electrode lines are connected to the ground side, and all drive switches 31 through 34 connected to the positive electrode lines are connected to the ground side, and thus the charge accumulated in the parasitic capacitors of all light-emitting elements E11 through E4n are discharged.
Once all light-emitting elements have been completely reset, scanning is shifted to the negative electrode line B2 to address the light-emitting elements E22 and E32 as shown in FIG. 5.
That is, the negative electrode line B2 is connected to the ground potential, the negative electrode lines B1 and B3 through Bn are also connected with the reverse bias voltage sources 41 and 43 through 4n, the positive electrode lines A2 and A3 to which the light-emitting elements E22 and E32 are connected are connected to the constant-current sources 22 and 23, and the remaining positive electrode lines A1 and A4 are connected to the ground potential.
As mentioned above, at the instant the scan switches 11 through 1n and drive switches 31 through 34 are switched over, the potential of the positive electrode lines A2 and A3 becomes approximately equal to V1 (more precisely nxe2x88x921/nxc2x7V1), and the voltage across both ends of the light-emitting elements E22 and E32 becomes a forward bias voltage approximately equal to the light emission specifying voltage Vx. Hence, the light-emitting elements E22 and E32 are quickly charged by the current from a plurality of routes shown with arrows in FIG. 5, and then are allowed to shift to a steady state of light emission shown in FIG. 6 instantaneously. In FIG. 6, the driving current supplied by the constant-current sources 22 and 23 flows only into the light-emitting elements E22 and E32 respectively, so that the light-emitting elements E22 and E32 are allowed to emit light with a desired instantaneous luminance Lx.
In the conventional driving method mentioned above, the problem relating to the rate in rise of light emission was eliminated. However, there still was a problem that power consumption increases since the charge accumulated in light-emitting elements is to be discharged completely each time scanning is shifted. Furthermore, the possibility of losing the display quality of images is developed due to the provision of the non-light emission period of a reset period at each time of scanning.
An object of the present invention is to provide a light-emitting display device with low power consumption and the driving method therefor. Another object is to improve display quality.
According to a first aspect of the present invention, in the driving method of a light-emitting display wherein light-emitting elements are connected to the intersections of positive electrode lines and negative electrode lines arranged in a matrix, either one of the positive electrode lines or the negative electrode lines are employed as scan lines with the other employed as drive lines; while scanning the scan lines, drive sources are connected to desired drive lines in synchronization with the scanning, whereby allowing the light-emitting elements connected to the intersections of the scan lines and drive lines to emit light, during a reset period after a scan period for scanning an arbitrary scan line is complete and before scanning the following scan line is started, a first reset voltage is applied to all of the scan lines and a second reset voltage that is greater than the first reset voltage is applied to all of the drive lines.
According to another aspect of the present invention, the difference between the second reset voltage and the first voltage is set to be lower than the light emission threshold voltage of the light-emitting element.
According to still another aspect of the present invention, the drive lines are connectable to either the drive source or a second reset voltage source for providing the second reset voltage, and the scan lines are connectable to either a first reset voltage source for providing the first reset voltage or a reverse bias voltage source for providing a predetermined reverse bias voltage.
According to still another aspect of the present invention, the first reset voltage source provides the ground potential.
According to still another aspect of the present invention, the reverse bias voltage source is almost the same as the voltage value determined by subtracting the second reset voltage from the light emission specifying voltage of a light-emitting element.
According to still another aspect of the present invention, during the reset period, all of the drive lines are connected to the second reset voltage source and all of the scan lines are connected to the first reset voltage source.
According to still another aspect of the present invention, during the scan period, scan lines to be scanned are connected to the first reset voltage source, scan lines not to be scanned are connected to the reverse bias voltage source, drive lines to be driven are connected to the drive sources, and drive lines not to be driven are connected to the second reset voltage source.
According to still another aspect of the present invention, the drive lines are connectable to either one of the drive sources, the second reset voltage source for providing the second reset voltage, or grounding means for providing the ground potential, the scan lines are connectable to either the first reset voltage source for providing the first reset voltage or the reverse bias voltage source for providing a predetermined reverse bias voltage.
According to still another aspect of the present invention, the first reset voltage source provides the ground potential.
According to still another aspect of the present invention, the reverse bias voltage source has almost the same voltage as the light emission specifying voltage of light-emitting elements.
According to still another aspect of the present invention, during the reset period, all of the drive lines are connected to the second reset voltage source and all of the scan lines are connected to the first reset voltage source.
According to still another aspect of the present invention, during the scan period, scan lines to be scanned are connected to the first reset voltage source, scan lines not to be scanned are connected to the reverse bias voltage source, drive lines to be driven are connected to the drive sources, and drive lines not to be driven are connected to the grounding means.
According to still another aspect of the present invention, the light-emitting elements are organic EL elements.
According to still another aspect of the present invention, the drive sources are constant-current sources.
According to still another aspect of the present invention, in a light-emitting display device in which light-emitting elements are connected to intersections of positive electrode lines and negative electrode lines arranged in a matrix, either one of the positive electrode lines or the negative electrode lines are employed as scan lines with the other employed as drive lines, a scan period during which drive sources are connected to desired drive lines while scanning the scan lines in synchronization with the scan and thus the light-emitting elements connected to the intersections of the scan lines and drive lines are lit, and a reset period for providing reset voltage for light-emitting elements are alternately repeated for display by light emission, the light-emitting display device comprises: scan switch means for enabling either of grounding means for providing a ground potential or a reverse bias voltage source for providing a predetermined reverse bias voltage to connect to each of the scan lines; drive switch means for enabling either of the drive source or reset voltage sources for providing the reset voltage to connect to each of the drive lines; and control means for controlling the switching of the scan switch means and the drive switch means in accordance with light emission data being inputted.
According to still another aspect of the present invention, the reset voltage is set to be lower than the light emission threshold voltage of the light-emitting elements.
According to still another aspect of the present invention, the reverse bias voltage source has almost the same voltage as the voltage determined by subtracting the reset voltage from the light emission specifying voltage of light-emitting elements.
According to still another aspect of the present invention, during the reset period, all of the scan switch means are connected to the grounding means and the drive switch means are connected to the reset voltage source.
According to still another aspect of the present invention, during the scan period, the scan switch means to be scanned are connected to the grounding means, the scan switch means not to be scanned are connected to the reverse bias voltage sources, the drive switch means to be driven are connected to the drive sources, and the drive switch means not to be driven are connected to the reset voltage sources.
According to still another aspect of the present invention, the drive switch means allow for selectively connecting to either one of the drive sources, the reset voltage sources, or grounding means for providing the ground potential.
According to still another aspect of the present invention, the voltage of the reverse bias voltage source is set to be almost the same as the light emission specifying voltage of the light-emitting elements.
According to still another aspect of the present invention, during the reset period, all of scan switch means are connected to the grounding means and the drive switch means are connected to the reset voltage sources.
According to still another aspect of the present invention, during the scan period, the scan switch means to be scanned are connected to the grounding means, the scan switch means not to be scanned are connected to the reverse bias voltage sources, the drive switch means to be driven are connected to the drive sources, and the drive switch means not to be driven are connected to the grounding means.
According to still another aspect of the present invention, the light-emitting elements are organic EL elements.
According to still another aspect of the present invention, the drive sources are constant-current sources.